Published on Web 01/26/2007
Framework-Catenation Isomerism in Metal-Organic Frameworks and Its
Impact on Hydrogen Uptake
Shengqian Ma,† Daofeng Sun,† Michael Ambrogio,† Jacqueline A. Fillinger,† Sean Parkin,‡ and
Hong-Cai Zhou*,†
Department of Chemistry & Biochemistry, Miami UniVersity, Oxford, Ohio 45056, and Department of Chemistry,
UniVersity of Kentucky, Lexington, Kentucky 40506
Received October 17, 2006; E-mail: zhouh@muohio.edu
Recent studies have focused on metal-organic frameworks
(MOFs) with high hydrogen uptake1 in order to reach the 2010
DOE targets for on-board vehicular hydrogen storage.2 Strategies
such as using pore sizes comparable to hydrogen molecules3 and
introducing coordinatively unsaturated metal centers (UMCs)3b,c,4
have been explored. Recently, we have reported a biomimetic
approach to UMCs utilizing entatic metal centers (EMCs).5 We
have also demonstrated in both PCN-63c (porous coordination networks) and PCN-95 that interpenetration is an important factor
contributing to their respective hydrogen uptake. However, interpenetration (or framework catenation) as an independent criterion
has never been resolved from other factors. Normally in a given
metal-ligand combination, either an interpenetrated network or
a non-interpenetrated network is favored, not both. Conceptually,
an interpenetrated MOF and its non-interpenetrated counterpart
can be viewed as a supramolecular pair of stereoisomers. In reality,
however, such framework-catenation isomerism has never been
deliberately explored prior to the present report. To study the precise role of catenation in hydrogen uptake, using oxalate as a
template, we have made the non-interpenetrated counterpart of
PCN-6 (PCN-6′).
At solvothermal conditions, a reaction between Cu(NO3)2‚2.5H2O
and H3TATB in the presence of oxalic acid afforded turquoise
octahedral crystals of PCN-6′ (yield: 60%). TATB represents
4,4′,4′′-s-triazine-2,4,6-triyl-tribenzoate. The reaction was run in
dimethylacetamide (DMA) at 75 °C. The overall formula of PCN6′ is Cu6(H2O)6(TATB)4‚DMA‚12H2O, determined by X-ray crystallography, elemental analysis, and thermogravimetric analysis
(TGA).
Singe-crystal X-ray6 diffraction studies reveal that PCN-6′
crystallizes in cubic space group Fm-3m. It is isostructural with
HKUST-1,7 mesoMOF-1,8 and with a single net of the interpenetrated PCN-6.3c In PCN-6′, dicopper tetracarboxylate paddlewheel
SBUs (secondary building units) are linked by TATB bridges. Each
SBU connects four TATB ligands, and each TATB binds three
SBUs to form a Td-octahedron (Figure 1b), which has idealized Td
symmetry with four ligands covering alternating triangular faces
of the octahedron and an SBU occupying each vertex. Eight such
Td-octahedra occupy the eight vertices of a cube to form a
cuboctahedron through corner sharing with idealized Oh symmetry
(Figure 1a).3c The average diameter of the void inside the
cuboctahedron is 30.32 Å. Every cuboctahedron connects eight Tdoctahedra via face-sharing, and each Td-octahedron links four
cuboctahedra to form a three-dimensional framework with a twisted
boracite net topology (Figure 1c). Open square channels from all
three orthogonal directions are identical in size and are 15.16 ×
15.16 Å2 or 21.44 × 21.44 Å2 along the edges or diagonals,
†
‡
Miami University.
University of Kentucky.
1858
9
J. AM. CHEM. SOC. 2007, 129, 1858-1859
Figure 1. (a) Cuboctahedral cage; (b) Td-octahedral cage; (c) a view of
the packing of PCN-6′ from the [001] direction; (d) a view of the
cuboctahedral net from the [111] direction of PCN-6′; (e) space-filling model
of the non-interpenetrated net in PCN-6′; (f) two interpenetrated nets in
PCN-6. The large spheres shown in graphics a, b, and d represent void
inside the cages.
respectively (atom to atom distance). Alternatively the structure
can be described as four honeycomb nets connected by the SBUs
at the center of the hexagonal-edges; the TATB ligands occupy
the corners of each hexagon and each hexagon is in a chair
conformation (Figure 1d).
The structure of PCN-6 (space group R-3m) can be reproduced
by two identical interpenetrated nets of PCN-6′, the second being
generated by translation of the first by c/5 (c represents the c-axis
in PCN-6) along [111] direction of PCN-6′. PCN-6′ and PCN-6
are thus catenation isomers (Figure 1e,f). To the best of our
knowledge, this is among the first pairs of such catenation isomers.3b
Variables in the synthetic procedures of PCN-6 and PCN-6′
include temperature, solvent, and template addition. Controlled
experiments were performed to confirm that template addition is
the only factor determining the final topology. Without the addition
of oxalic acid, the reaction between Cu(NO3)2‚2.5H2O and H3TATB
in DMA, dimethylformamide (DMF), or diethylformamide (DEF)
at 75 °C or 120 °C (with DMSO as solvent) led to the formation
of PCN-6. In contrast, with oxalic-acid addition the same reaction
in DMA, DMF, or DEF at 75 °C or 120 °C (DMSO) gives PCN6′. Thus, only template addition can account for the presence or
absence of catenation.
To test the general pertinence of this finding, another large
trigonal-planar ligand HTB (for s-heptazine tribenzoate)9 was
employed to react with Cu(NO3)2‚2.5H2O under reaction conditions
similar to those of PCN-6 and PCN-6′. As expected, an interpenetrated MOF isostructural with PCN-6 (MOF-HTB) was
obtained without the addition of oxalic acid, while a noninterpenetrated MOF isostructural with PCN-6′ (MOF-HTB′) was
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Figure 2. Gas sorption isotherms (77 K) of PCN-6′ and PCN-6 activated
at 50 °C for N2 (left) and H2 (right).
formed in the presence of oxalic acid. The mechanistic details of
this remarkable templating effect are still under investigation.
Both PCN-6′ and PCN-6 exhibit permanent porosity, confirmed
by gas sorption (Figure 2) and powder X-ray diffraction (PXRD)
studies (Figure S5). In contrast, both HTB MOFs collapse upon
guest solvent removal presumably owing to the instability of the
larger open channels in the two HTB MOFs. The N2 adsorption
isotherm of PCN-6′ (Figure 2a) indicates typical Type-I sorption
behavior, with a Langmuir surface area of 2700 m2/g (pore volume
1.045 mL/g). This is lower than the Langmuir surface area of PCN-6
(3800 m2/g, pore volume 1.453 mL/g), although PCN-6′ has a
higher solvent-accessible volume (86%, calculated using PLATON10) than that of PCN-6 (74%). Thus, catenation has led to 41%
increase in Langmuir surface area. This counter-intuitive increase
can be attributed to that the new adsorption sites are formed by the
catenation as well as the small pores formed as a result of catenation
may strengthen the overall interaction between gas molecules and
the pore walls therefore increase the apparent surface area, as
predicted by a recent theoretical simulation.11 If open channels are
blocked as a result of catenation, however, the overall surface area
may drop significantly.
It is important to study the independent contribution of catenation
to hydrogen uptake. In the catenation isomer pair of PCN-6 and
PCN-6′, the contributions from interpenetration and UMCs must
be separated. TGA studies on the two MOFs suggest that activation
at 50 °C will remove only guest molecules. To expose the UMCs,
the samples must be heated at 150 °C to remove the axial aqua
ligands on the Cu centers. A facile way to resolve the contributions
by UMCs and catenation to hydrogen uptake is to measure the
hydrogen adsorption isotherms of samples activated at the two
temperatures.
Hydrogen adsorption studies revealed that PCN-6′ activated at
50 °C can adsorb 1.35 wt % (volumetric uptake of 3.94 kg/m3,
calculated density: 0.292 g/cm3) hydrogen at 760 Torr and 77 K
(Figure 2b), significantly lower than that of PCN-6 activated at
the same temperature (1.74 wt %; volumetric uptake of 9.19 kg/
m3, calculated density: 0.528 g/cm3). Hence, catenation has led to
a 133% of enhancement in volumetric (29% in gravimetric)
hydrogen uptake. Previously we have reported that after activation
at 150 °C, PCN-6 can adsorb 1.9 wt % hydrogen (at 760 Torr and
77 K), a 10% improvement over that of PCN-6 activated at 50 °C
(Figure 3). Similarly, after activation at 150 °C, PCN-6′ can adsorb
1.62 wt % hydrogen, a 20% increase from that of PCN-6′ activated
at 50 °C (Figure 3). These further improvements upon activation
at 150 °C can only be attributed to UMCs. The smaller improvement
of hydrogen uptake upon UMC activation in PCN-6 than that in
PCN-6′ suggests that most of the UMCs in PCN-6 are blocked as
a result of catenation (observed from the crystal structure), while
the UMCs in PCN-6′ are open.
In summary, using an unprecedented templating strategy, new
catenation isomer-pairs can be synthesized predictably using copper
Figure 3. H2 sorption isotherms of PCN-6′ (left) and PCN-6 (right)
activated at 50 and 150 °C.
paddlewheel SBUs and two trigonal-planar ligands (TATB and
HTB) developed in our lab. Gas sorption studies on the isomer
pair using TATB have revealed that catenation leads to a 41%
improvement of Langmuir surface area and a 133% increase in
volumetric hydrogen uptake (29% increase in gravimetric). The
resolution of the contributions from UMCs and catenation to the
hydrogen uptake of a MOF is unprecedented. Future work will focus
on investigating the mechanistic details of oxalate-templating and
applying this strategy to other systems.
Acknowledgment. This work was supported by the National
Science Foundation (Grant CHE-0449634) and Miami University.
H.C.Z. also acknowledges the Research Corporation for a Research
Innovation Award and a Cottrell Scholar Award. The diffractometer
was funded by NSF Grant CHE-0319176 (S.P.).
Supporting Information Available: Detailed experimental procedures, X-ray structural data (cif files), PXRD patterns, TGA graphics.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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